Titanium article



3,7432% Patented Jan. 22, 1963 3,i74,829 TITANIUM ARTILE Virgil F. Novy,Altadena, and Craig G. Kirkpatrick, Granada Hills, Calif, assignors toNuelear Corporation of America, Inc, Denville, Ni, a corporation ofDeiaware N Drawing. Filed Feb. 11, 1959, Ser. No. 814,655 4 Qlaims. (Cl.148-31.5)

This invention relates to titanium alloy articles including rare earthmetal, and to methods for making these articles.

In the metallurgy of titanium, the carbon, oxygen, hydrogen and nitrogenpresent in titanium is known as the interstitial content of thetitanium. As discussed at pages 337 and following of a text entitled,Titanium by A. C. McQuillan et al., Butterworth Scientific Publications,London, 1956, relatively small amounts, less than 1 or 2 percent, ofthese interstitially-soluble impurity elements in titanium have a greatstrengthening and hardening effect. However, they also have the adverseeffect of producing brittleness in the titanium and of making itdifiicult to work. Furthermore, a high interstitial content greatlyincreases the corrosion susceptibility of titanium surfaces.

Accordingly, an important object of the present invention is theavoidance of the adverse effects of interstitial content in titaniumwhile retaining the desired increase in strength.

In accordance with the present invention, this object is achieved by theaddition of a relatively small percentage of a rare earth metal such asgadolinium to titanium having some interstitial impurities. Thepercentage of the added metal may range from .05 to 2.5 percent,preferably 0.l to 0.5 percent, by weight of the alloy, depending on theinterstitial content of the alloy. When the rare earth metal containingtitanium alloy is heated to an appropriate temperature as discussedbelow, and quenched as by immersion in water, an essentially puretitanium coating is formed on the surface of the alloy. In addition, thecore of the alloy shape is greatly strengthened by the heat treatment.

One advantage of this process involves the high degree of ductility ofthe titanium alloy, which can be easily rolled, shaped, formed, orotherwise worked without heating, prior to heat treating. Followingtransformation type hardening produced by the heat treatment, the corehas greatly increased strength, stiffness and hardness. In addition,maximum corrosion resistance is accomplished by the formation of a thinlayer of pure titanium along all the exposed surfaces of the alloyshape. When a sheet is subject to this treatment, the end product is alaminated sheet including a hard, high-strength, center ply, covered bytwo outside layers of ductile, highly corrosion-resistant titanium.

Other objects and advantages, and various features of our invention willbe apparent from a consideration of the following detailed description.

In one example of the present invention, a titanium alloy buttonincluding 0.18 percent by weight of gadolinium was formed by are meltingin accordance with conventional techniques. The interstitial content ofthe titanium employed in making the button showed nitrogen less than0.003 percent, hydrogen 0.007 percent, and oxygen 0.19 percent. Arcmelting has the effect of increasing the oxygen content somewhat. Thealloy button was cold rolled to a sheet thickness 0.35". The microstruoture of the resulting cold rolled button, as revealed in aphotomicrograph, showed the uniform striations in the longitudinaldirection of rolling which are typical of cold rolled structures.

Samples of the rolled sheet were then sealed in silica glass tubing ofthe type designated Vicor, and made by Corning Glass Works. The sampleswere then heat treated at a temperature of 1785 F. for 24 hours, andwater quenched. Heating should take place in an inert atmosphere orvacuum. An inert atmosphere for heat treating may be provided by otherknown techniques instead of by sealing in quartz tubing.

The microstructure of the heat treated sample was then examined byconventional metallographic techniques. The photomicrograph's revealed atitanium coating which was developed at the surface of the sample uponquenching. The thickness of the coating was determined to be 0.0026inch.

The structure of the base metal at the center of the sample indicates amartensitic type of transformation from the beta to the alpha structureupon quenching. The coating of essentially pure titanium, however, isnot subject to the martensitic transformation and therefore has thenormal alpha structure, which is relatively ductile. The uniform grainstructure of the heat treated core, as contrasted with the original coldrolled structure, indicated complete recrystallization.

The samples formed as described above have very high shear and tensilestrengths. In addition, the elongation approached zero, the ductility ofthe core is very low, and the hardness measured Rockwell 20C.

With regard to the materials which may be employed, gadolinium hasproved to be particularly effective. In addition, however, good titaniumcoatings have been formed on titanium based alloys including yttrium,erbium and lanthanum. These materials are all rare earth metals having aclose-packed hexagonal crystal form, which is also characteristic oftitanium. Other rare earth metals having a close-packed hexagonalcrystal structure may also be employed. These additional elementsinclude cerium, praseodymium, neodymium, terbium, dysprosium, holmium,thulium, and lutetium. Scandium and yttrium, atomic numbers 21 and 39,occur together with the rare earths in nature, and are also group IIIAelements. They are therefore generally included in the term rare earths,and are so included when this term is employed in the presentspecification and claims. Scandium and yttrium also have the desirableclosepacked hexagonal crystal form.

The length of time and the temperature of the required heat treatmentdepends on factors such as the sample size and the composition of thesample. The important thing is that the temperature be high enough, andthe holding time be long enough so that substantially all of thetitanium is transformed into the beta structure. With pure titanium thetransition to the beta structure begins in the neighborhood of 882.5 C.which corresponds to about 1620 F. The transformation from the alpha tothe beta structure starts at temperatures which increase rapidly beyond882.5 C. for samples of increasing interstitial content. With largersize pieces and high interstitial content, therefore, it is evident thathigher temperatures and longer holding times are required in order totransform all of the titanium from the alpha to the beta structure.

One advantage of the titanium clad titanium alloy involves the highcorrosion resistance of pure titanium metal. In this regard, thecorrosion resistance of titanium metal decreases with increasinginterstitial content. On the other hand, however, high interstitialcontent is desirable for the purpose of obtaining good mechanicalproperties such as hardness and stiffness. The heat treatment describedabove provides a hard core with high interstitial content, and a puretitanium outer surface. Accordingly, the shapes produced by heattreatment are adrnirably suited for high strength metal parts which aresubject to salt water corrosion or similar exposure. In this regard, animport-ant field of use for the titanium coated shapes would be as abase for anodes employed in cathodic protection systems.

It may also be noted that the McQuillan text cited above providesconsiderable background regarding the strength and the melting point oftitanium with various concentrations of interstitially-solubleimpurities. Through reference to this or comparable text material,processes for producing titanium clad shapes having a core with thedesired properties may readily be determined.

Even without the rapid quench from the beta structure, the alloysincluding small percentages of rare earth material 'such as gadoliniumshow considerable improvement over normal titanium metal. Without therapid quench, the alloys are ductile and can be cold rolled from acasting into very thin sheets Without intermediate annealing processes.

It is to be understood that the above described arrangements areillustrative of the application of the principles of the invention.Numerous other arrangements may be devised by those skilled in the artWithout departing from the spirit and scope of the invention.

What is Claimed is:

1. A metal shape comprising a core of titanium alloyed with from 0.05 to2.5 percent by Weight of gadolinium metal, and a layer of essentiallypure titanium on the surface of said shape.

References Qited in the file of this patent UNITED STATES PATENTS1,819,722 Sugimura et a1. Aug. 18, 1931 2,766,113 Chisholm et a1 Oct. 9,1956 2,940,163 Davies June 14, 1960 OTHER REFERENCES Handbook onTitanium Metal, 7th edition, published by Titanium Metals Corp. ofAmerica, New York (pp. 16 and 17 relied upon).

Titanium (McQuillan et al.), publ. by Butterworths ScientificPublications, London, 1956 (pp. 314-317 relied on).

Constitution of Binary Alloys (Hansen), publ. by McGraw-Hill Book Co.,New York, 1958 (p. 463 relied on).

2. AN ALLOY CONSISTING ESSENTIALLY OF TITANIUM AND FROM .05 TO 2.5PERCENT BY WEIGHT OF GADOLINIUM.